**Fig.2 The cat and the block.**
The Fig.2 shows a cat playing with 3 identical solid
blocks. Each square drawn on the box is 1 cm^{2}.

Each block has a mass of 24g. This may be considered
as 24 gram weights. As Force (weight) is mg. this is equal to 0.2 N. *Find the pressure under each block.*

**Worked out example.**

We shall calculate the pressure under ‘A’.

Calculating pressure
under ‘A’.

Weight of block .
. . = 0.2 N.

Area under the block =
2 cm^{2}.

Pressure under the block
= 0.2/2 = __0.1 N.cm__^{-2}.

*If you are clever you will work out the
pressure under ‘B’ and “C’.*

**Liquid Pressure**

In a liquid, pressure can be measured at any depth.
Pressure increases directly with the height or depth. Irrespective of the
volume of the liquid or the area at the base, at a particular depth the
pressure remains the same. But the density of the liquid affects the pressure.

The measuring cylinders given contain three different
liquids. Their densities are given. Let us calculate the liquid pressure at the
bottom.

**Fig,2.Calculating the pressure at any depth.**

__Calculating the liquid
pressure for oil.__

1. The
height of oil . .
. = 3.0 cm.

Density of oil .
. . =
0.7 g.cm-3

Gravitational
attraction. . = 0.0098 N per gram. (1 Kg weight = 9.8 N)

Formula for liquid
pressure. = **height x density x g.**

= 3 x0.7 x 0.01, N cm^{-2} = __0.02 N cm__^{-2}

__Calculating the liquid
pressure for water.__

2. Formula for liquid pressure **= ***h d
g.*

*
*= 4x 1x0.01
= __0.04 cm__^{-2}

*In the same manner you should be able to calculate the
pressure for the Juice,*

**Gas
Pressure.**

This is usually measured by the height of a mercury
column or a water column that can balance the gas pressure.

Atmosphere consists of a gaseous mixture. So it should
exert a pressure on us from all sides. This can be conveniently measured by
using a liquid of high density. **The density of mercury is 13.6 times that of
water**; so it is the most suitable for the purpose.

A thin glass tube is completely filled with mercury
and inverted in an open cup of the same liquid. There should be a good
knowledge of the technique to fill the tube so that there wont be any air
bubbles inside. Then the tube is inverted taking care not to allow any air
enter. The height of the column will change according to the out side pressure.

Normal atmospheric pressure at sea level is 76 cm of
mercury.

*Try converting this to Pascals. (Pa).*

A pressure of 1N on an area of 1m^{2} is = 1 Pa.

This is called a **Barometer**.

**What’s
your Lung Pressure?**

For you to sing or even speak you have to blow out air
at a certain pressure. The lung pressure is usually measured by a column of
water.

A colour is added to the water to make it easier to see the levels. At first the levels, inside the tube and the bottle would be equal. When you blow the liquid level will go up. After yo have made your best effort , measure the difference in levels, That will be your maximum lung pressure.